Method of hydrocarbon recovery by in-situ combustion



United States Patent Oflice Patented Aug. 19, 1969 8 Claims ABSTRACT OF THE DISCLOSURE Water is injected through a first well into an oil bearing formation in an amount equal to between lpercent of the pore volume and that required for breakthrough into a second well (theoretically 100 percent of the pore volume). Subsequently, oil recovery by in situ combustion is conducted in the formation between the two wells, preferably in a direction moving from the second well to the first well. The technique can be applied with multiple wells.

This application is a continuation-in-part Ser. No. 408,386, Method of Hydrocarbon Recovery filed Nov.

2, 1964, now abandoned, which is a continuation-inpart of application Ser. No. 134,859, Method of Hydrocarbon Recovery filed Aug. 30, 1961, now abandoned.

This invention relates to the recovery ofv hydrocarbons from subterranean formations wherein the naturally occurring producing forces have been substantially. or completely depleted. More particularly, it is a method of recovery wherein a hydrocarbon bearing formation is subjected to more complete recovery to produce greater amounts of the hydrocarbons by the injection of water into the formation followed by conducting in situ combustion throughout the same formation.

The recovery of hydrocarbons from a subterranean reservoir is conventionally accomplished atthe outset by utilizing the naturally-occurring energy occurring within the formation in the form of pressure exerted by water or gas, or a combination thereof, which is then normally followed with recovery by energy developed by pumping facilities. The foregoing recovery is generally referred to as primary recovery which is normally inadequate to produce a substantial portion of the hydrocarbons that remain in place after it becomes economically unfeasible to continue such operations. As a result of the inability to produce major portions of the hydrocarbons within a formation by primary recovery, methods of recovery for these hydrocarbons remaining in the formation were developed which are referred to as secondary recovery methods. The secondary recovery of hydrocarbons from the formation is accomplished with an injected fluid or combination of fluids, and with or without the application of heat to the formation. Typicalfluids conventionally injected into the formation for the displacement of the hydrocarbons are gaseous hydrocarbons for a gas pressurization or gas flood drive, liquefied low molecular weight normally gaseous hydrocarbons or carbon dioxide for a miscible flood drive, water for a water flood drive, and appropriate combinations of these. The application of heat can be accomplished by heating, the material to be injected at the surface prior to injection, or by heating the formation itself by in situ combustion, that is igniting and combusting a portion of the hydrocarbons in place to create a thermal front as a fire flood, whereby the heat developed reduces the viscosity of the hydrocarbons and the front is propagated through the formation to displace the hydrocarbons therefrom. Further the prior art teaches the recovery method whereby a formation is subjected to a combination of floods, particularly a fire flood followed by a water flood whereby a hot water drive is accomplished to recover the hydrocarbons not previously produced. These methods of recovery are satisfactory for producing portions of the hydrocarbons not recovered by primary efforts, but there are many formations wherein the application of the foregoing is inapplicable for technical or economical reasons.

I An object of'this invention is to provide a method of secondary recovery whereby additional amounts of hydrocarbons are recovered from subterranean formations wherein the primary recovery methods are no longer applicable.

A further object of this invention is to provide a method of secondary recovery whereby production of hydrocarbons from a partially depleted reservoir is more economically advantageous.

Further objects and advantages of this invention will be apparent from the following detailed description.

Stated broadly the present invention is a method for the secondary recovery of hydrocarbons whereby the hydrocarbon-bearing subterranean formation, either completely or an intermediate portion thereof, is initially subjected to a water flood. Thereafter permeability is established in the waterflooded formation and a fire flood subsequently is conducted in the waterflooded formation in a direction counter to the direction of flow of the original water flood.

The method of the present invention can be applied to any formation traversed by a plurality of wells sufficient to provide a pattern, such as a five-spot, a line, or another arrangement of wells specifically adapted to a particular formation. Initially wells must be drilled, or drilled wells incorporated, in a suitable pattern, whereafter certain of these wells are determined to be appropriate as the injection wells and the recovery wells for the initial phase of this recovery operation.

The initial phase of the present invention is the injection of water through injection wells into the formation'in a conventional manner whereby a portion of the hydrocarbons in the formation contacted by the water is displaced toward the recovery wells. This water injection can be applied to the complete vertical distance of the formation, or the water can be selectively injected into an intermediate portion of the formation. The water injection step is done primarily to condition the reservoir for the subsequent phase of fire flooding or conducting in situ combustion throughout the formation, though an appreciable amount of hydrocarbons can be recovered from the formation during the water flooding phase. The flooding media, water, has a greater initial areal sweep efliciency than any other media, that is, water inherently has the least fingering or channeling characteristics, so extends into greater volumes of the formation than any other displacement medium, to contact and fill a greater volume of the pore space of the formation. However, the water is less suitable from the standpoint of oil displacement efficiency, due to the complete immiscibility of water and the liquid hydrocarbons which causes the water to enter the oil saturated portion of the formation to a lesser degree than other displacement media. Water effectively displaces a lesser portion of the oil contacted in its greater sweep by the hydraulic pressure mechanism and the natural tendency of water to flow through the portion of the pore space saturated with oil due to the capillary action of the inherent water film on the formation particles. Viscosity differences in the formation fluids and water reduce the efficiency of the hydraulic mechanism and cause a substantial portion of the oil to be bypassed during the initial phase of water flood operations. The overall effect of the water injection phase is to displace about to percent of the oil from oil-bearing ortions of the formation, depending upon the viscosity of the crude oil. More importantly, the injection of water reduces the oil saturation within the formation over a much greater area than could be accomplished with any other medium due to the lower tendency for water to finger, as compared to gas or LPG, etc. The injection of water in accordance with the preferable embodiment of this invention is done with limited amount of water, rather than the several pore volumes needed to achieve a complete water flood throughout the entire formation. The minimum volume of injected water is an amount in excess of 1 percent of the formation pore volume, and the upper limit is that amount required for breakthrough of the water into the recovery wells, theoretically 100 percent of the pore volume. The injection of less than 100 percent is recommended to allow th combustion operation to be conducted at an earlier point; and, though it is possible to inject more than 1 pore volume, it is without commensurate advantage. The water injection can be continued, as during conventional water flooding, until there is a breakthrough of water into one or more recovery wells indicating the completion of a sweep by the water injected into the formation encompassed by the pattern, or even continued thereafter until the water-oil ratio is so economically unattractive that further water injection is unfeasible.

The initial phase of this invention, water flooding displacement, results in the recovery of additional amounts of hydrocarbons, but the primary purpose is to condition the formation for the latter phase of this invention which is fire flooding of the formation. Upon termination of the initial water phase injection, the formation is in condition for conducting an in situ combustion process therein. The first step is creation of the requisite gas saturation within the formation by the injection of gaseous fluids, such as methane, natural gas, combustion gas or air, or mixtures thereof, into the formation. The gas saturation is achieved for the individual formation at that point wherein the cumulative gas injected is suflicient to facilitate the injection of a subsequent amount of oxygen-containing gas to provide for the initiation and propagation of a burning zone through the formation. This gas can be injected through the same injection wells utilized during the preliminary water flood operation. or the wells utilized for injection can be altered to take advantage of any particular formation configuration of the water deposit occurring during the injection of water effected previously. The well or wells to be used for gaseous fluid injection are those wells which traverse a portion of the formation wherein there is water saturation, so it is foreseeable that the injection or the recovery wells of the initial water flood phase can be used as gaseous fluid injection wells. Therefore, the displacement front of this phase of fire flooding can be moved through the formation in the reverse direction of the water flood front, in the event that the injection and recovery wells are reversed for the fire flood operation.

The injection of gaseous fluids into the formation is conducted in a conventional manner until sufficient gas saturation is achieved to provide the necessary gas permeability which is a function of saturation. Thereafter oxygen-containing gas is injected into the formation in an amount sufiicient to establish combustion conditions within the formation during and subsequent to ignition or initiation of the in situ combustion. Injection of the gaseous fluids into the portion of the formation containing water improves the in situ combustion operation economics because the gas saturation is more readily accomplished since the gaseous fluids more readily saturate those portions of the formation which contain water rather than viscous oil. Further, the presence of the water deposit in a greater portion of the formation, due to the relatively high areal sweep efliciency of water, causes the gaseous fluids to saturate the same greater portion which would normally be impossible, due to the inherent fingering characteristics of injected gases. The areal sweep efliciency of gas injection in a hydrocarbon bearing reservoir which has not been subjected to a water flood is from about 30 to about 50 percent, and only the comparable portion of the formation within the pattern is capable of being subjected to in situ combustion. The areal efficiency of water injection in the same reservoir is from about to about percent; and, since the injection of gaseous fluids into the resultant water deposit is coincident with the water deposit, the distribution of gas saturation within the pattern is correspondingly increased, that is, the portion of the formation capable of being subjected to in situ combustion is increased about 200 percent.

In addition to the increased areal efliciency for the conduct of an in situ combustion operation within a formation, the capacity for injecting oxygen containing gas into the formation previously water flooded is also increased with respect to the original reservoir capacity. To illustrate this point, a Brea sandstone core measuring three inches in diameter and twelve inches in length was placed in a plastic tube and incorporated in a flow system wherein tests were conducted to determine the effect of water and oil saturations upon the gas injection capacities of the core with regard to gas saturation and permeability to gas, and the time requirements for the accomplishment of the desired gas saturation. The core had the following properties: absolute air permeability of millidarcies (md.); porosity of 18.4 percent; and pore volume of 252 cubic centimeters. A series of laboratory tests were conducted on this core which were directed to injecting a non-hydrocarbon gas, nitrogen, through the core while it was saturated with water and a liquid hydrocarbon, providing results tabulated in the following Tables IIII:

TABLE 1 Gas injected Gas saturation (percent pore volume) (cumulative pore volumes) Before water flood After Water flood TABLE 11 Gas injected Gas permeability (md.)

(cumulative pore volumes) Before water flood After water flood 0 0 1. 4 3. 1 1. 9 4. 3 2. 4 5. 3 3. 1 6. 2 3. 7 t3. 9 4. 5 7. 6 5. 2 8. 3 6. 1 9. 1 ti. 9 2). 8 7. 8 10. u 7 12. 2

TABLE III Gas saturation (percent Before After Before After water water water water Time (min) flood flood flood flood The foregoing laboratory data indicate the marked effectiveness of injecting a gas into a formation containing liquid oil which has been water flooded previously thereto, rather-than attempting to inject the gas without such flooding, because of the greater displacement ability of the gas through the water as opposed to, the liquid hydrocarbon. 1

The application of this data to a field situation having the following characteristics was calculated:

D Formation depth feet 1000 d Well Spacing (5 acre 5 spot) do 330 h Formation thickness do 50 r Well bore radius inches 4 T Formation temperature F 70 k Absolute permeability md 160 S Initial gas saturation percent 4 K Initial gas permeability md 0.2 I Porosity percent 18.4

An extension of the laboratory data by calculation to a formation having the above characteristics is presented in the following Tables IV and V:

TABLE IV Gas injection rate (MM s.c.f./day) Time (days) Before water flood After water flood TABLE V Gas Injected Gas injection rate (MM s.c.f./day) (cumulative volume MM s.c.f.) Before water flood After water flood In the above tables, the water flood was calculated to be achievable by injecting water at a rate of 750 b./d. until 18,000 barrels is injected which would require injection for a period of 24 days. These tables presenting information as to gas injection rates provide the necessary data to determine the time and expense required to initiate an in situ combustion operation Within the formation. Assuming a formation having the characteristics set forth above, it would be preferable to obtain an oxygen-contain- 6 ing gas injection rate of 2,000,000 mcfd. before the initiation of combustion.

The information set forth in Table IV, as derived from calculations based on laboratory data set forth in the previous tables, indicates that the injection rate of 2,000,000 mcfd. necessary for ignition within the formation would 'be reached in about days in the case of a formation which has been water flooded, and about 300 days in the case of a formation which has not been water flooded. Thus it is evident that applying the method of recovery of the present invention will reduce the time required to accomplish combustion gas saturation in the formation by a factor in excess of 2. Further a reference to Table V reflects the difference in the cumulative volumes of gaseous fluid which would have to be injected to attain the required injection rate of 2,000,000 mcfd, that is, about 125,000,000 s.c.f. for the formation subjected to a water flood as opposed to about 212,000,000 s.c.f. for the formation not subjected to water flood. Thus the difference in the cumulative gas injection requirements for the present method is less than that required by the conventional method by a factor in the order of 2.

Therefore the present inventive method materially increases the efliciency of the gas injection operation and concurrently reduces the amount of gas which must be injected to raise the gas saturation of the formation to the required amount necessary to obtain ignition of the in situ combustion within the formation. Upon achieving the gas saturation by injecting gaseous fluids and oxygen-containing gas into the formation by the use of conventional injection equipment, the combustion is initiated in the formation adjacent one or more of the injection wells by the use of conventional means such as a mechanical or electrical heater, introduction of chemical reactants, etc.

Subsequent to ignition of the hydrocarbons in the formation, the injection of oxygen-containing gas is continued to propagate the front outwardly from the injection well toward the recovery well as in a conventional fire flood. The in situ combustion can be conducted in any conventional manner, such as disclosed in US. Patent No. 2,642,943, whereby substantially all the hydrocarbons remaining in the reservoir are displaced therefrom. The fire flood of the present method achieves greater recovery than the conventional fire flood, however, because the combustion is propagated throughout only those portions of the reservoir wherein the necessary gas saturation has been accomplished. The increased recoveries result from a utilization of areal sweep efliciency normally unobtainable in a fire flood because of the initial injection of water which has the greater areal sweep eificiency that is carried forward to the fire flood step since the gaseous fluids follow essentially the same configuration as the injected water deposit in establishing the required gas saturation for the combustion or fire flood. This increased recovery is due to the greater volume of the formation combusted which will be in the order of 70 percent rather than the conventional 35 percent, based on the difference in the areal sweep efficiencies of gas and water as set forth hereinabove.

A modification of the present inventive method is the application of the initial step of water flooding to only a portion of the hydrocarbon-bearing formation, whereby an intermediate zone in the vertically disposed layers of the formation is subjected to hydrocarbon displacement 5 by the injected water. The location of this zone can be selected by the use of proper well completion techniques and individual formation characteristics, such as vertical permeability barriers, so as to maximize the beneficial effects of subsequent operations.

Water injection will usually be terminated, due to high water-oil ratio or water breakthrough at the recovery wells. Thereafter, requisite gas saturation and permeability is accomplished as above in the water-bearing portion, and the oxygen-containing gas is injected into this intermediate portion. In situ combustion is initiated as before and conducted in a conventional manner in this intermediate zone within the formation. The result is that the major combustion occurs in the intermediate portion of the formation because the gas saturation is greatest therein and hydrocarbons are recovered from the portions of the formation adjacent the intermediate zone, while avoiding intense combustion in these adjacent zones to such a degree as to utilize and conserve substantial amounts of heat normally lost to adjacent formations which do not contain hydrocarbons. The application of this method in the modified form as applicable to thick formations can be extended in particular circumstances, such as extremely viscous deposits, by subsequently further flooding the complete formation or specifically the adjacent portions thereof, with a conventional fluid such as water to utilize the heat therein to recover any residual hydrocarbons initially by-passed in the adjacent portions of the formation. The areal sweep efliciency is substantially the the same as in the instance of water flooding the complete formation, but there is even greater advantage derived with regard to conservation of the combustionsupporting gas requirements and injection time. This is primarily the result of burning only a fraction of the total vertical section of the reservoir, thereby permitting a reduction in the compressor investment required to deplete the reservoir by combustion drive and increasing the rate of response at production wells by promoting early breakthrough of the combustion front.

The recovery of hydrocarbons by the method of the present invention must be considered to be complete upon the occurrence of a gas-oil ratio which is economically unattractive, thereafter the operation is terminated by stopping further injection of oxygen-containing gas into the formation. Termination can be accomplished at any point that the individual recovery conditions require by merely stopping injection of the oxygen-containing gas.

It has been the practice in the past to utilize various additives in water employed in the secondary recovery of hydrocarbons to aid in the recovery process. In addition, other additives have been contemplated, for example, in U. S. Patent No. 3,024,840 there is added to the water a dispersible organic compound which undergoes thermal decomposition to yield a solid carbonaceous residue. This residue late-r serves as fuel for an in situ combustion operation. As employed herein, the term flooding agent is intended to cover essentially water, although other materials, additives, etc., which aid in the flooding operation, can be included. It is not intended that this term, however, include materials such as those set forth in the Allen patent, or any other materials whose principal function is not to aid in the water flooding operation.

It is obvious that many other variations and modifications can be made in the method of this invention without departing from the spirit and scope thereof as defined in the appended claims.

What is claimed is:

1. A method for the recovery of hydrocarbons from a subterranean, hydrocarbon bearing formation traversed by a first and a second well which comprises:

(a) injecting water only into an intermediate zone in the vertically disposed layers of said formation through said first well in an amount in excess of one percent of the pore volume of said intermediate zone;

(b) terminating the injection of said water at least upon breakthrough of said water into said second well;

(c) injecting a gaseous fluid only into said intermediate zone through one of said first and second wellsin an amount substantially saturating said intermediate zone;

(d) terminating the injection of said fluid;

(e) injecting an oxygen-containing gas only into said intermediate zone through said second well in an amount to form combustion conditions in said inter mediate zone-about said second well;

(f) initiating in situ combustion only in said intermediate zone adjacent said second well;

(g) propagating said combustion through said intermediate zone toward said first well;

(h) displacing said hydrocarbons from said intermediate zone into said first well;

(i) recovering said hydrocarbons from said first well to the surface; and

(j) terminating the injection of said gas upon completing the recovery of the hydrocarbons of said formation.

2. The method of claim 1 wherein step (c) comprises injecting a gaseous fluid into said formation through said first well.

3. The method of claim 1 wherein step (c) comprises injecting a gaseous fluid into said formation through said second well.

4. The method of claim 1 further characterized in that said gaseous fluid of step (c) is other than oxygen-containing.

5. The method of claim 1 further characterized in that said gaseous fluid of step (c) is combustion inert.

6. The method of claim 1 further characterized in that said oxygen-containing gas in step (e) is air.

7. The method of claim 1 further characterized in that a plurality of wells is substituted for said first well.

8. The method of claim 1 further characterized in that a plurality of Wells is substituted for said second well.

References Cited UNITED STATES PATENTS 2,642,943 6/1953 Smith et a1. 166-11 2,973,811 3/1961 Rogers 16611 X 3,019,837 2/1962 Marx et a1. l6611 3,024,841 3/1962 Willman 166l1 3,170,515 2/1965 Willman 166l1 3,193,008 7/1965 Moore 166l1 3,246,693 4/1966 Crider 1661l X STEPHEN I. NOVOSAD, Primary Examiner US. Cl. X.R. 

